Structural schematic diagram of pointer multimeter
A set of cylindrical iron core is embedded in the circular magnetic steel. Outside the cylindrical shape is a movable aluminum frame. Inside the aluminum frame is a coil made of fine enameled wire. There are sharp needles at both ends of the coil, a hairspring and a zero-adjustable rod. Its flexibility determines the sensitivity of the watch head, which moves like a watch's pendulum hand. The two ends of the coil are a positive pole and the other negative pole. The red line represents the positive pole + and the black line represents the negative pole 1. It is an aluminum frame that can rotate around an axis. Two flat coil springs and a pointer are mounted on the axis of the aluminum frame. The two ends of the coil are respectively connected to the two coil springs, and the current to be measured enters the coil through the springs. On the two poles of the horseshoe magnet, there is a pole piece with a cylindrical inner wall, and a fixed cylindrical iron core inside the aluminum frame. The function of the pole piece and the iron core is to make the magnetic induction lines between them along the radial direction And distributed evenly along the circumference.
When the coil moves in the magnetic field, no matter where it turns, its plane is parallel to the magnetic field lines. When the current passes through the coil, both sides of the coil parallel to the axis are subjected to magnetic force, and the function of these two forces is to make the coil rotate. When the variable meter coil rotates, the helical spring is twisted to produce a force that hinders the coil rotation, and its torque force increases with the increase of the coil rotation angle. When this hindering effect increases to cancel the rotating effect of the magnetic field force, the coil stops rotating. According to the principle that the same sex repels each other and the opposite sex attracts each other according to the characteristics of the magnetic field, once the microampere-level coil magnetic field is reversed, the magnetic field it generates will become the opposite magnetic field interacting with each other and deflecting in the opposite direction. Furthermore, the current force through the coil is proportional to the current, so the greater the current in the coil, the greater the rotation of the magnetic field force, and the greater the deflection angle between the coil and the pointer. Therefore, according to the deflection angle of the pointer, it can be Knowing the strength of the measured current, when the direction of the current in the coil changes, the direction of the magnetic field force will also change accordingly, and the deflection direction of the pointer will also change accordingly. Therefore, according to the deflection direction of the pointer, the direction of the measured current can be known.
There is no reverse bias in the measurement of AC voltage, and the measurement of DC voltage: put a switch of the multimeter in the DC voltage block V of the appropriate range, and the "+" test lead (red test lead) is connected to the high potential, and the "-" test lead (Black test lead) is connected to the low potential, that is, let the current flow in from the "+" test lead and flow out from the "- test lead. If the test lead is reversed, the pointer of the meter head will deflect in the opposite direction, and it is easy to bend the pointer.
When measuring DC current, place a switch of the multimeter on a suitable range from 50uA to 500mA. The range selection and reading method of current is the same as that of voltage. When measuring, the circuit must be disconnected first, and then the multimeter is sent to the circuit under test according to the direction of the current from "+" to "-", that is, the current flows in from the red test lead and flows out from the black test lead. If the multimeter is connected in parallel with the load by mistake, the internal resistance of the meter head is very small, which will cause a short circuit and burn the meter. The reading method is as follows: actual value = indicated value × range / full deviation.
